I. Field of the Invention
This invention relates to gas chromatography and more particularly relates to an apparatus and method for gas detection using a quenching-resistant multiple flame photometric detector.
II. Description of the Related Art
For gas chromatography (GC), there exist a variety of detectors that can provide information regarding sample composition. A flame-based device that is very widely used in the selective analysis of sulfur and phosphorus compounds is the relatively robust and inexpensive flame photometric detector (FPD). First developed by Brody and Chaney several decades ago (Brody and Chaney, 1966), the FPD has evolved into a reliable tool that is still prevalently used in many important areas such as the analysis of pesticides (Li et al., 2008; Fuentes et al., 2008; Chen et al., 2008), petroleum (Wang et al., 2009; Zhao et al., 2008; Baruah and Khare, 2007), and chemical warfare agents (Seto et al., 2007; Logan et al., 2006; Kendler et al., 2005).
One of the strengths of the FPD is its ability to selectively detect lower quantities of sulfur and phosphorus analytes relative to other hydrocarbons (Brody and Chaney, 1966; Sevcik and Thao, 1975). However, it also has some disadvantages. For instance, although linear operating regions can be accessed (Aue and Sun, 1993), conventional FPD sulfur response is often pseudo-quadratic due to the predominant chemiluminescence of S2 produced in the hydrogen-rich flame (Farwell and Barinaga, 1986). Further, the yield of this species is dependent upon burner design, gas flows, analyte concentration and molecular structure. Also, its response can vary among individual compounds, leading to a relatively non-uniform response factor over a broad range of sulfur analytes (Sugiyama et al., 1973; Burnett et al., 1978).
A further drawback is that analyte chemiluminescence in the FPD is typically quenched in the presence of even moderate amounts of co-eluting hydrocarbons (Dressler, 1986; McGuffin and Novotny, 1981; Pearson and Hines, 1977; Clay et al., 1977). As such, this can complicate the analysis of complex samples, since the complete separation of analytes from all matrix components is often impractical. Although the mechanism for this response quenching is still not fully established, several possibilities have been suggested (Kalontrov et al., 1995; Aue and Sun, 1993; Sugiyama et al., 1973; Cheskis et al., 1993). Nonetheless, the resulting FPD signal erosion observed can present serious problems for the analyst.
Of the various kindred FPD devices reported over the years, occasionally those based upon unconventional combustion dynamics, such as the pulsed-FPD (Kalontrov et al., 1995; Cheskis et al., 1993) and the reactive flow detector (Thurbide and Aue, 1995), have been demonstrated to reduce the effect of hydrocarbon quenching. Perhaps the most widely investigated device in this regard is the dual-flame FPD (dFPD) (Rupprecht and Phillips, 1969; Patterson et al., 1978; Patterson, 1978; Machino, 1996; Koizumi and Suzuki, 1991; Tolosa et cd., 1991; Poole, 2003; Poole and Schuette, 1984; Ferguson and Luke, 1979; Tuan et al., 1994), which is based upon two flames placed in series. In the dFPD, the lower flame is normally optimized to oxidize hydrocarbons toward carbon dioxide formation, while the upper flame is optimized to produce analyte chemiluminescence for measurement. The result is a reduction in analyte response quenching and a more uniform, reproducible response (Poole, 2003).
When first developed by Rupprecht and Phillips (1969), the consecutive oxygen-rich then hydrogen-rich dFPD flame environments employed produced negligible hydrocarbon quenching toward sulfur chemiluminescence. In their report, they concluded that quenching is greatly reduced when carbon is present as carbon dioxide versus a less oxidized hydrocarbon (Rupprecht and Phillips, 1969). Subsequently, Patterson et al. (1978) expanded upon this by designing a dFPD that easily converts to a single flame FPD as needed (Patterson et al., 1978; Patterson, 1978). This dFPD model demonstrated several advantages including similar optimal flame gas flows for sulfur and phosphorus detection and a more uniform response, as well as the reduced hydrocarbon quenching that was noted earlier (Rupprecht and Phillips, 1969). Thus, the dFPD offers some useful properties and is often regarded as a quenching-free device for practical applications (Kalontrov et al., 1995), particularly those dealing with elevated hydrocarbon levels (Patterson, 1978).
Despite these benefits, however, the dFPD has some significant disadvantages. For example, dFPD burner designs are often somewhat bulky and complex, and in some cases may require a rather elaborate ignition sequence to initially establish the dual flames (Rupprecht and Phillips, 1969; Patterson et al., 1978). In particular, these features can inhibit incorporation of this technology into miniaturized/portable analytical devices. However, of much greater concern, the dFPD is widely reported to produce significantly reduced detector sensitivity relative to a conventional FPD (Rupprecht and Phillips, 1969; Patterson, 1978; Poole, 2003; Poole and Schuette, 1984; Ferguson and Luke, 1979; Tuan et al., 1994). Given the noted advantages of the dFPD, it would be beneficial if these problems could be overcome. There is a need in the industry for a compact simple detector that would easily establish consecutive flames.
Recently, a micro counter-current FPD (μFPD) method was introduced that is based upon relatively small opposing flows of hydrogen and oxygen (Thurbide et al., 2004; Thurbide and Anderson, 2003; Thurbide and Hayward, 2004; Hayward and Thurbide, 2006). As such, this arrangement can produce a very tiny (˜30 nL) flame that resides ‘upside down’ on a stainless steel capillary delivering oxygen in a counter-flowing stream of hydrogen. Once established, the micro flame is capable of producing both chemiluminscent and ionization responses that are respectively very similar to the conventional Flame Photometric and Flame Ionization (FID) Detectors (Thurbide and Hayward, 2004; Hayward and Thurbide, 2007; Hayward and Thurbide, 2008). As a result, this device can potentially provide a relatively simple, robust, and sensitive FPD method in a micro analytical format (Thurbide and Hayward, 2004; Hayward and Thurbide, 2008). Table 1 summarizes its properties relative to an FPD and dFPD.
TABLE 1Characteristics of Single, Dual, and Micro-Flame Photometric DetectorsPhotometricDetectorAdvantagesDisadvantagesSingle FlameGood sensitivitySignificant quenching(FPD)Simple operationLimited response uniformityLimited reproducibilityLimited portabilityDual FlameReduced quenchingReduced sensitivity(dFPD)Improved responseIncreased complexityuniformityLimited portabilityImproved reproducibilityMicro FlameGood sensitivitySignificant quenching(μFPD)Simple operationLimited response uniformityGood portabilityLimited reproducibility
The referenced shortcomings are not intended to be exhaustive, but rather are among many that tend to impair the effectiveness of previously known techniques in flame photometric detectors; however, those mentioned here are sufficient to demonstrate that the methodologies appearing in the art have not been satisfactory and that a significant need exists for the techniques described and claimed in this disclosure.